## **Core Concept**
The sensitivity of a diagnostic test is a measure of its ability to correctly identify those with the disease (true positive rate). It is calculated as the number of true positive results divided by the sum of true positives and false negatives. In the context of this question, it refers to the test's ability to correctly identify fetuses with trisomy 21 (Down's syndrome).

## **Why the Correct Answer is Right**
To calculate the sensitivity of the new test for detecting trisomy 21, we use the formula: Sensitivity = True Positives / (True Positives + False Negatives). From the given data: True Positives = 100 (fetuses with trisomy 21 who tested positive), and False Negatives = 100 (fetuses with trisomy 21 who tested negative). Therefore, Sensitivity = 100 / (100 + 100) = 100 / 200 = 0.5 or 50%. This means that the test correctly identifies 50% of the fetuses with trisomy 21.

## **Why Each Wrong Option is Incorrect**
- **Option A:** If we calculate the value, it does not match 50%, so it is incorrect without further specification.
- **Option B:** Similarly, if the value does not match 50%, it is incorrect.
- **Option D:** If this option does not equal 50%, then it is incorrect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the sensitivity and specificity of a test are independent of the prevalence of the disease in the population being tested. However, the predictive values (positive and negative) do depend on the prevalence. For screening tests, especially for conditions like Down's syndrome, a balance between sensitivity and specificity is crucial. A test with high sensitivity is particularly useful for ruling out a condition when it is negative.

## **Correct Answer:** B. 50%.